1. Field of the Invention
The present invention relates to an ink jet recording apparatus in which a carriage configured to move along a recording medium is equipped with a detection unit capable of detecting whether the recording medium is in an abnormal state by detecting a distance to the recording medium.
2. Description of the Related Art
As one type of recording apparatus such as a printer, a facsimile, a copying machine for recording an image based on image information on a sheet-shaped recording medium such as paper, there is an ink jet recording apparatus which records an image by discharging ink from a recording head onto a recording medium. A typical ink jet recording apparatus is a serial-type ink jet recording apparatus including a recording head mounted on a carriage capable of reciprocating in a main scanning direction intersecting with a conveying direction of recording medium, and performing a recording operation by discharging ink from the recording head in synchronization with a movement of the carriage. In this serial-type recording method, recording on an entire recording medium is performed by alternately repeating a recording operation of recording one line while the carriage is being moved, and a conveyance operation of conveying the recording medium by a predetermined pitch. Examples of recording medium include a cut sheet divided into a predetermined size, and a continuous sheet such as roll paper. A recording apparatus using a continuous sheet cuts the continuous sheet at the rear end of a recorded image of a predetermined amount by a cutter unit, and discharges the portion of the sheet with the image recorded thereon.
If abnormal conveyance such as sheet uplift occurs at the time of conveyance of a recording medium through a recording unit, this may result in incorrect recording on the recording medium. For example, some types (characteristics) of recording medium or some conditions of ambient environment such as a temperature and humidity may cause a cutter unit to be caught by a recording medium when the cutter unit cuts the recording medium, which may lead to sheet uplift, then resulting in generation of a scrape between the recording head and the recording medium. If the sheet uplift is significant enough to apply a large load to the carriage and cutter, the recording operation is immediately stopped to prevent the recording head from being heavily damaged. However, in some cases, sheet uplift only slightly increases the load applied to the carriage and the cutter, while scrapes of the recording head are being accumulated. In such a case, this abnormality cannot be distinguished from a load fluctuation in a normal cutting operation, so that the scrape of the recording head cannot be detected. Then, if the recording head is repeatedly damaged from the scrapes, not only correct recording may become impossible but also the recording head may be broken.
Japanese Patent Application Laid-Open No. 2005-015132 discusses a technique of determining an abnormality in a recording medium being conveyed based on a detection result about the width of the recording medium and the distance from the recording medium to a recording head with use of a distance measurement sensor capable of detecting the distance from the recording head on a carriage to the recording medium. Further, Japanese Patent Application Laid-Open No. 2004-074710 discusses a technique of detecting the distance to a recording medium and the width of the recording medium with use of a sensor mounted on a carriage to detect an abnormality in the recording medium being conveyed based on the detection result.
However, since the technique discussed in Japanese Patent Application Laid-Open No. 2005-015132 is a method of detecting a flap on a tractor apparatus on which the edges of a continuous sheet on the both sides are placed, this technique can detect only a sheet uplift state at the edges of the sheet on the both sides, and therefore cannot detect a sheet uplift state at the center position of the sheet. On the other hand, the technique discussed in Japanese Patent Application Laid-Open No. 2004-074710 detects an abnormal state at an arbitrary position or a plurality of positions of a sheet with use of the distance measurement sensor mounted on the carriage. Then, this technique immediately determines that the conveyance state is abnormal when the detection result exceeds a predetermined range. These features of the method discussed in Japanese Patent Application Laid-Open No. 2004-074710 lead to the following problems. When an abnormal state such as sheet uplift is searched at a plurality of positions or in the entire region of a sheet prior to a printing operation, a sheet having a large size increases a time spent on detection movement of the carriage, thereby reducing the throughput even when the conveyance is in a normal state. Further, even though detection is performed by moving the carriage during cutting of a sheet to prevent a reduction in the throughput, since the sheet changes its posture relative to the carriage, output of the sensor fluctuates, thereby reducing the detection accuracy. Further, although sheet uplift may be alleviated through a sheet push-out operation, sheet uplift is searched at a position where the sheet behaves differently from the time of printing, thereby reducing the detection accuracy. As a result, even conveyance that is not abnormal may be determined as a conveyance failure.